1. Field of the Invention
The present invention relates to a piezoelectric vibration device system including a piezoelectric vibration device that has a piezoelectric element, and an electronics apparatus including such a piezoelectric vibration device system.
2. Description of the Related Art
In the related art, fuel cells have been widely put to practical use for industrial or household electric power generators, or for power sources of artificial satellites or spaceships because they have high power generation efficiencies without exhausting harmful materials. In addition, fuel cells used for power sources of vehicles such as passenger cars, buses, and trucks have been significantly developed in recent years. Such fuel cells can be categorized into several types—such as an alkaline solution type, a phosphoric acid type, a molten carbonate type, a solid oxide type, and a direct methanol type. Among others, a direct methanol solid polymer electrolyte fuel cell (DMFC (direct methanol fuel cell) for short hereinafter) has been extensively researched and developed to be used for a small portable fuel battery because the DMFC can be highly energy-densified by using methanol as a supply source of hydrogen for fuel, and also it can be made small because it can be realized without a reformer.
In a DMFC, an MEA (membrane electrode assembly), which is a unit fuel cell composed of a solid polymer electrolyte membrane and two electrodes (gas diffusion electrodes), is used, where the membrane and the two electrodes are assembled in an all-in-one structure with the membrane disposed between the two electrodes. In this case, one of the gas diffusion electrodes is set as a fuel electrode (negative electrode), and the other is set as an oxygen electrode (positive electrode). The surface of the fuel electrode is fed with methanol as fuel, with the result that the methanol dissolves into hydrogen ions (protons) and electrons. On the other hand, the surface of the oxygen electrode (positive electrode) is fed with air as oxidizing gas. The hydrogen ions produced at the fuel electrode pass across the solid polymer electrolyte membrane, and the electrons produced at the fuel electrode are transported to the oxygen electrode through an external load or the like connected between the fuel electrode and the oxygen electrode. At the oxygen electrode, the oxygen in the air reacts with the hydrogen ions and the electrons to produce water. Owing to the above-described electrochemical reaction, the DMFC has an electromotive force between the oxygen electrode and the fuel electrode, with the result that a current flows through the external load.
In the DMFC, two types of supply methods that supply methanol to the fuel electrode are proposed—one is a liquid fuel supply type (method where liquid fuel (methanol solution) is directly supplied to the fuel electrode), and the other is a gas fuel supply type (method where vaporized fuel obtained by vaporizing liquid methanol is supplied to the fuel electrode). A scroll pump, a diffuser pump, a cascade pump, a gear pump, a screw pump, a diaphragm pump, a piston pump, a plunger pump or the like has been commonly used in the above supply methods where fuel such as methanol is supplied to the fuel electrode. However, because the above-mentioned pumps consume a significant amount of electric power and the downsizing of a DMFC with one of the above pumps installed is difficult, a piezoelectric pump (used as a fuel pump) having a piezoelectric element has also come to be used recently for supplying the fuel to the fuel electrode.
On the other hand, a rotary pump, a mechanical booster pump, or the like has been commonly used for supplying oxidizing gas (air) to the oxygen electrode. Recently, however, a piezoelectric pump (used as an air pump or a blower) having a piezoelectric element has also come to be used for supplying the oxidizing gas.
In order to control the behavior of such a piezoelectric pump having a piezoelectric element, the vibration frequency of the piezoelectric element is controlled, which is disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 05-184169 and 07-245971.